Syrian activists have shared horrific images of the aftermath of an airstrike on a school in Idlib province that killed as many as 35 people.

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At least 22 children died in the attack on the school in the village of Hass, in Maarat al-Numan, northern Idlib, according to the Syrian Observatory for Human Rights. The London-based group initially placed the death toll at 14, but had warned it would rise.

Pictures taken from the scene showed numerous casualties.

Abd, who did not want his second name used, was near the village when the airstrike occurred.

He told BuzzFeed News, via a messaging service, that when he got there it was “horrible.”

“There was blood and damage everywhere,” he said. Abd, who was studying at university before the war, also visited the school, which he said had been hit just as the school day was finishing — and reduced to nothing. “It [the airstrike] was for killing the children only,” he claimed.

He described seeing members of the White Helmets, a voluntary rescue organisation working in rebel-held areas of Syria, trying to pull someone out from underneath a destroyed house.

"Many mothers are still trying to find their children,” said Yasser Trraf, a 22-year old activist from the village. “We don’t have any armed group in this village, we only have organizations to support children and refugees and schools.”

Other reports, which emerged later, also indicated the school was targeted in what is known as a "double-tap" strike, whereby a building is bombed once, then again when rescuers arrive to pull people from the rubble. Double-tap strikes have gained a vicious notoriety for their use in the Syrian conflict.

Aid organisations have condemned the fresh attack on a civilian building. Unicef said that if deliberate, the strike was a war crime.

Anthony Lake, executive director of the international NGO, said in a statement to BuzzFeed News: "This is a tragedy. It is an outrage. And if deliberate, it is a war crime."

Lake said the atrocity could be the deadliest attack on a school since the war began more than five years ago. "Children lost forever to their families, teachers lost forever to their students, one more scar on Syria’s future," he said.

"When will the world’s revulsion at such barbarity be matched by insistence that this must stop?"

An activist posted online that the local imam was telling parents whose children had not returned from school to come to the mosque, where they could “identify them among bodies."

Videos purportedly showed the moment the munitions struck, as the airstrike came down over the village before detonating with a large flame.

Video also reportedly taken from the village showed men stumbling through rubble towards a large smoke plume, and traumatized children reeling from the effects of the strike being hustled into a medic’s van.

Idlib is home to one of the remaining rebel strongholds, and has been frequently targeted by the Syrian government and Russian forces in support of the regime.

Rose Troup Buchanan is a reporter for BuzzFeed News and is based in London.